Category-specific attention for animals reflects
ancestral priorities, not expertise
Joshua New*†‡, Leda Cosmides*, and John Tooby*
*Center for Evolutionary Psychology, University of California, Santa Barbara, CA 93106; and †Department of Psychology, Yale University,
New Haven, CT 06520
Edited by Gordon H. Orians, University of Washington, Seattle, WA, and approved August 10, 2007 (received for review May 3, 2007)
Visual attention mechanisms are known to select information to
process based on current goals, personal relevance, and lowerlevel features. Here we present evidence that human visual attention also includes a high-level category-specialized system that
monitors animals in an ongoing manner. Exposed to alternations
between complex natural scenes and duplicates with a single
change (a change-detection paradigm), subjects are substantially
faster and more accurate at detecting changes in animals relative
to changes in all tested categories of inanimate objects, even
vehicles, which they have been trained for years to monitor for
sudden life-or-death changes in trajectory. This animate monitoring bias could not be accounted for by differences in lower-level
visual characteristics, how interesting the target objects were,
experience, or expertise, implicating mechanisms that evolved to
direct attention differentially to objects by virtue of their membership in ancestrally important categories, regardless of their
current utility.
animacy 兩 category specificity 兩 domain specificity 兩
evolutionary psychology 兩 visual attention
V
isual attention is an umbrella term for the set of operations
that select some portions of a scene, rather than others, for
more extensive processing. These operations evolved because
some categories of information in the visual environment were
likely to be more important or time-sensitive than others for
activities that contributed to an organism’s survival or reproduction. The selection criteria that direct visual attention can be
categorized by their origin: (i) goal-derived: criteria activated
volitionally in response to a transient internally represented goal;
(ii) ancestrally derived: criteria so generally useful for a species,
generation after generation, that natural selection favored mechanisms that cause them to develop in a species-typical manner;
and (iii) expertise-derived: criteria extracted during ontogeny by
evolved mechanisms specialized for detecting which perceptual
cues predict information that enhances task performance.
These three types of criteria may also interact; for example,
differential experience or temporary goals could calibrate or
elaborate ancestrally derived criteria built into the attentional
architecture.
The ways in which human attention can be affected by goals
and expertise have been extensively investigated. Indeed, humans are zoologically unique in the extent to which we evolved
to engage in behavior tailored to achieve situation-specific goals
as a regular part of our subsistence and sociality (1, 2). Among
our foraging ancestors, improvising solutions in response to the
distinctive features of situations would have benefited from the
existence of goal-driven voluntary attentional mechanisms. As
predicted by such a view, otherwise arbitrary but task-relevant
objects command more attention than task-irrelevant ones (3),
and expertise in a task domain shifts attention to more tasksignificant objects (4), features (5), and locations (6).
In contrast, attentional selection criteria that evolved in
response to the payoffs inherent in the structure of the ancestral
world have been less systematically explored. Yet, the rapid
identification of the semantic category to which an object
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belongs (e.g., animal, plant, person, tool, terrain) and what its
presence in the scene signifies [e.g., predatory danger, food
(prey), offspring at risk] would have been central to solving many
ancestral adaptive problems. That is, stably and cumulatively
across hundreds of thousands of generations, attention allocated
to different semantic categories would have returned different
average informational payoffs. From this perspective, it would be
odd to find that attention to objects was designed to be deployed
in a category-neutral way. Yet there has been comparatively little
research into whether some semantic categories spontaneously
recruit more attention than others, and whether such recruitment might be based on evolved prioritization. Most exceptions
have studied attention and responses to highly social information
such as faces (7, 8), eye gaze (9), hand gestures (10), and stylized
human outlines (stick drawings and silhouettes) (11).
The Animate Monitoring Hypothesis
For ancestral hunter-gatherers immersed in a rich biotic environment, non-human and human animals would have been the
two most consequential time-sensitive categories to monitor on
an ongoing basis (12). As family, friends, potential mates, and
adversaries, humans afforded social opportunities and dangers.
Information about non-human animals was also of critical
importance to our foraging ancestors. Non-human animals were
predators on humans; food when they strayed close enough to be
worth pursuing; dangers when surprised or threatened by virtue
of their venom, horns, claws, mass, strength, or propensity to
charge; or sources of information about other animals or plants
that were hidden or occluded; etc. Not only were animals (human
and non-human) vital features of the visual environment, but
they change their status far more frequently than plants, artifacts, or features of the terrain. Animals can change their minds,
behavior, trajectory, or location in a fraction of a second, making
their frequent reinspection as indispensable as their initial
detection.
For these reasons, we hypothesized that the human attention
system evolved to reliably develop certain category-specific
selection criteria, including a set designed to differentially
monitor animals and humans. These should cause stronger
spontaneous recruitment of attention to humans and to nonhuman animals than to objects drawn from less time-sensitive or
vital categories (e.g., plants, mountains, artifacts). We call this
the animate monitoring hypothesis. Animate monitoring algorithms are hypothesized to have coevolved alongside goal-driven
Author contributions: J.N., L.C., and J.T. designed research; J.N. performed research; J.N.
and L.C. analyzed data; and J.N., L.C., and J.T. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Abbreviations: CD, change detection; Exp n, Experiment n; RT, reaction time.
See Commentary on page 16396.
‡To
whom correspondence should be addressed. E-mail: [email protected].
This article contains supporting information online at www.pnas.org/cgi/content/full/
0703913104/DC1.
© 2007 by The National Academy of Sciences of the USA
www.pnas.org兾cgi兾doi兾10.1073兾pnas.0703913104
voluntary processes that focus attention on task-relevant objects,
providing the voluntary system with one of several interrupt
circuits made necessary by a surprising world. These algorithms
should operate automatically and autonomously from executive
function, so that important changes in non-humans and humans
can be detected rapidly, even when they are unexpected or
irrelevant to current goals or activities. Hence, we propose that
animate inputs will recruit visual attention in a way that is less
context-, goal-, expertise-, and state-dependent than other inputs. Although increasingly focused attention may increasingly
screen out task-irrelevant stimuli, such exclusion should affect
human and animal stimuli less than members of other categories.
In particular, subjects’ attention should display the predicted
animate monitoring bias in the absence of instructions to look for
animals or humans and regardless of their relevance to the task
or to subjects’ goals.
The counterhypothesis is that visual attention contains no
mechanisms designed to differentially allocate attention on the
basis of the semantic category of the input. This means there
should be no mechanisms that evolved to deploy attention
differentially to animate targets, and therefore no animate
monitoring bias should be found. If, nevertheless, evidence of
such a bias were to be found, the fallback hypothesis would be
that such an effect would be the result of expertise: that is,
starting with an equipotential attentional system, ontogenetic
training would accrete attentional biases as a function of differential experience with the stimulus inputs and their ontogenetic
importance. We will call this the expertise hypothesis.
Assessing Preferential Attention
Experiments show that viewers often fail to detect sizeable
changes in an image when these occur during very brief interruptions of sight, a phenomenon known as change blindness (13,
14). To explore the selection criteria implemented by attentional
mechanisms, we used the change detection (CD) paradigm (Fig.
1), in which viewers are asked to spot the difference between two
rapidly alternating scenes that are identical except for a change
to one object. The logic is straightforward: in a CD paradigm,
changes to more attended objects or regions in a complex natural
scene will be detected faster and more reliably than changes to
less-attended ones. By varying which features in a scene are
changed, one can learn the criteria by which visual attention
mechanisms select objects for further processing. In a CD
New et al.
Tests and Predictions
If, as hypothesized, the human attentional architecture includes
evolved mechanisms designed to differentially direct attention to
both human and non-human animals, then, in a CD task using
complex natural scenes, we predict that: (i) changes to animals
(both human and non-human) will be detected more quickly
than changes to inanimate objects and (ii) changes to animals
will be detected more frequently than changes to inanimate
objects. By hypothesis, attention is differentially recruited to
animals by virtue of neural processes recognizing (at some level)
their category membership. The bias is category-driven. Therefore, (iii) although animals will be judged more interesting than
inanimate objects, detection rates will be better predicted by the
target’s category (animate or inanimate) than by how interesting
the targets are judged to be, and (iv) the detection advantage for
animate categories will not be due to lower-level perceptual
characteristics, such as visual complexity or high contrast.
According to the expertise counterhypothesis, any effects by
category will arise from differences in frequency of observation,
differential training with different categories, or the relative
importance during ontogeny of paying differential attention by
category. We selected vehicles as an evolutionarily novel contrast
category with which subjects have a great deal of experience;
which move and do so in a self-propelled fashion; and which
subjects have been trained from childhood as pedestrians and
drivers to differentially attend to because of the life-or-death
importance of anticipating their momentary shifts in trajectory.
In comparison, our subjects see and interact with non-human
animals far less often than with vehicles, and animals have little
practical significance to our subjects. Despite greater subject
expertise with vehicles, we predict that (v) the animate bias will
not be a consequence of ontogenetic exposure to things in
motion. In particular, although subjects see large numbers of
vehicles moving every day, changes to vehicles will not be
detected as quickly or reliably as changes to animals and people.
Finally, this study affords an opportunity to measure the
effects of expertise on visual attention. Subjects have a lifetime
of intensive training in attending to one species above all:
humans. In contrast, subjects have orders-of-magnitude less
experience attending to any other given species. The difference
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SEE COMMENTARY
PSYCHOLOGY
Diagram illustrating the sequence and timing of each trial in Exp 1–5.
EVOLUTION
Fig. 1.
experiment, subjects are instructed to detect changes, but they
are not given any task-specific goal that would direct their
attention to some kinds of objects over others. Thus, the CD
paradigm can be used to investigate how attention is deployed in
the absence of a voluntary goal-directed search (15). If the
animate bias hypothesis is correct, then change blindness will be
attenuated for animals and humans compared with other object
categories. This is because category-specific attention mechanisms will automatically check the status of animals and people
on an ongoing basis.
We adapted a standard CD task (14) to test for the predicted
category-specific biases (Fig. 1). The stimuli were color photographs of natural complex scenes (Fig. 2). For Experiments
(Exp) 1–4, 70 scenes with target objects from five semantic
categories were used (14 in each category): two animate (people
and animals) and three inanimate [plants; moveable/
manipulable artifacts designed for interaction with human
hands/body (e.g., stapler, wheelbarrow); fixed artifacts construable as topographical landmarks (e.g., windmill, house)]. These
categories were chosen because converging evidence from neuropsychology and cognitive development suggests each is associated with a functionally distinct neural substrate (16, 17). Each
involves an evolutionarily important category, but only the
animates require close visual monitoring. Target categories for
Exp 5 (96 scenes) were vehicles, artifacts that do not move on
their own, non-human animals, and people. [For details, see
supporting information (SI) Appendix 1].
Fig. 2. Sample stimuli with targets circled. Although they are small (measured in pixels), peripheral, and blend into the background, the human (A) and elephant
(E) were detected 100% of the time, and the hit rate for the tiny pigeon (B) was 91%. In contrast, average hit rates were 76% for the silo (C) and 67% for the
high-contrast mug in the foreground (F), yet both are substantially larger in pixels than the elephant and pigeon. The simple comparison between the elephant
and the minivan (D) is equally instructive. They occur in a similar visual background, yet changes to the high-contrast red minivan were detected only 72% of
the time (compared with the smaller low-contrast elephant’s 100% detection rate).
in performance between attention to humans and attention to
other animal species gives a measure of the importance of
expertise in training attention to animate inputs.
Results
Exp 1 was designed to test predictions i–iii (above) of the animate
bias hypothesis, Exp 2 was a replication of Exp 1, and Exp 3–5
were designed to test predictions iv–v.
The hit rate (percent correct) was used to assess accuracy,
because false alarms were so rare across the five experiments
(2% of all responses; SI Appendix 1.1). Reaction times (RTs) are
for hits.
Do Animals and People Recruit Preferential Attention? Yes. Changes
to animals and people were detected more often and more
quickly than changes to inanimate objects in Exp 1 and 2 (Fig.
3 A and B). More specifically, changes to animate targets
(animals and people) were detected faster than changes to
inanimate ones (plants, moveable artifacts, and fixed artifacts),
both in Exp 1 and its replication (Exp 2); animate vs. inanimate
target RTs: P ⫽ 10⫺10 and 10⫺15, respectively. Changes to
animate targets were detected 1–2 seconds faster than changes
to inanimate ones, and the effect size (r) associated with this
difference was large in both experiments (0.88 and 0.86).
The greater speed in detecting changes to animals and people
was not achieved at the expense of accuracy. On the contrary,
subjects were faster and more accurate for animate targets,
which elicited hit rates 21–25% points higher than inanimate
targets (Exp 1 and 2, r ⫽ 0.84 and 0.80; P ⫽ 10⫺8 and 10⫺10;
false-alarm rates were low, 0.92% and 1.6%). Overall, 89% of
changes to animate targets were correctly detected vs. 66% of
changes to inanimate ones. The animate advantage in speed and
accuracy remains strong, even when inanimates are compared
only to non-human animals (see Fig. 3; RT, r ⫽ 0.80 and 0.64,
P ⫽ 10⫺7 and 10⫺11; hits, r ⫽ 0.82 and 0.63, P ⱕ 0.0002).
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Following convention, we reported RTs for hits only. However, this measure fails to capture cases in which a change to the
target was missed entirely; missing a change is a more severe case
of ‘‘change blindness’’ than being slow to notice one. Subjects
were change-blind more often for inanimate targets than for
animate ones (miss rates, 34% inanimate vs. 11% animate).
Because this is not reflected in mean RTs for hits, the difference
between animate and inanimate RTs underestimates the animate attentional advantage. Moreover, mean RTs can mask
important differences in the time course of change detection.
Fig. 3 addresses these concerns by showing, for each category,
the time course of change detection. The relationship between
time elapsed and total number of changes detected is plotted.
Steeper slopes indicate earlier change detection; higher asymptotes mean more changes were eventually detected (i.e., less
change blindness). Consistent with the hypothesis that animals
and people should undergo incidental monitoring so that
changes in their location and state can be rapidly detected, the
curves for the two animate categories have steeper slopes and
asymptote at higher levels than those for the three inanimate
categories. Moreover, there appear to be attentional capture as
well as monitoring effects.
Attentional Capture. The animate and inanimate curves diverge
quickly: there were more hits for animate than for inanimate
targets even for the fastest responses, ones in which changes were
detected in ⬍1 second (Exp 1, hits 8.8% vs. 3.9%; P ⫽ 0.0025,
r ⫽ 0.52, no false alarms; Exp 2, hits, 3.8% vs. 1.6%, P ⫽ 0.002,
r ⫽ 0.48; one false alarm). This suggests that animates capture
attention in addition to eliciting more frequent monitoring. The
maximal difference between animate and inanimate detection
occurred at 3.5–4 elapsed seconds, a 33–37% point difference,
with an effect size of r ⬎ 0.93 (P values ⫽10⫺14).
Do Animals and People Receive Preferential Attention Because They
Are More ‘‘Interesting?’’ In CD studies, interesting items are
detected faster than uninteresting ones (14, 18). When a separate
New et al.
SEE COMMENTARY
ratings do not predict RTs once one statistically controls for
whether the target is animate (partial r ⫽ ⫺0.16, P ⫽ 0.20). In
contrast, animate targets elicit faster RTs, even after controlling
for interest ratings (partial r ⫽ ⫺0.41, P ⫽ 0.001; see SI Appendix
1.2). The same result holds for hit rates (animacy, partial r ⫽
0.37, P ⫽ 0.002; interest, partial r ⫽ 0.064, P ⫽ 0.60). Thus, the
animacy bias was not a side effect of animals and people being
more ‘‘interesting’’ targets, as judged by deliberative processes.
Fast, accurate change detection of animates results from a
category-driven process: animacy, not interest, predicts change
detection efficiency.
Fig. 3.
Changes to animals and people are detected faster and more
accurately than changes to plants and artifacts. Graphs show proportion of
changes detected as a function of time and semantic category. (Inset) Mean RT
for each category (people, animals, plants, moveable/manipulable artifacts,
and fixed artifacts). (A) Results for Exp 1. Animate targets: RT M ⫽ 3,034 msec
(SD, 882), hit rate M ⫽ 89.8% (SD, 7.4). Inanimate targets: RT M ⫽ 4,772 msec
(SD, 1,404), hit rate M ⫽ 64.9% (SD, 15.7). (B) Results for Exp 2. Animate
targets: RT M ⫽ 3,346 (SD, 893), hit rate M ⫽ 88.7% (SD, 8.0). Inanimate RT M ⫽
4,996 (SD, 1,284), hit rate M ⫽ 67.5% (SD, 16.5). (C) Results for Exp 5. RT:
animate M ⫽ 2,661 msec (SD, 770). Hit rate, animate vs. vehicle: 90.6% (SD, 7.8)
vs. 63.5% (SD, 18.8), P ⫽ 10⫺15.
group of subjects rated how interesting each target was (SI
Appendix 1), interest ratings correlated with animacy (r ⫽ 0.60,
P ⫽ 10⫺7). But does this explain the animate attention effect?
No. Although they were correlated with animacy, interest
New et al.
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EVOLUTION
effect found in Exp 1 and 2 reflect nothing more than a confound
in the stimuli? Given that lower-level features (e.g., color,
luminance) can affect attention in simple visual arrays (19) and
more natural and complex scenes (20), it is important to
eliminate the hypothesis that, in the particular stimuli we used,
these features differed for animate vs. inanimate targets.
Target luminance, size (pixels), and eccentricity were entered
into a multiple regression model; none predicted RT or accuracy, either across or within domains (P values range from 0.2 to
0.9). To eliminate counterhypotheses invoking any potential
lower-level feature(s), Exp 3 and 4 were conducted.
Inverting photos perfectly preserves their lower-level stimulus
properties but makes identifying the semantic category to which
a target belongs more difficult (8, 18, 21). Further, scene
inversion sizably reduces the detection of changes to highrelative to low-interest items (18) (but see ref. 22). If lower-level
properties are causing the animate attention advantage, then it
should appear even when photos are inverted. In contrast, if the
attentional bias is category-driven, then manipulations that
interfere with categorization but preserve lower-level perceptual features should eliminate the animate change-detection
advantage.
Exp 3 was identical to Exp 1 and 2, except the photos were
inverted. The procedure and stimuli for Exp 4 were also the
same, except target category identification was disrupted not by
inverting but by blurring each photo with a Gaussian blurring
function in Adobe Photoshop (see SI Appendix 2). This preserves
many lower-level characteristics (although not as perfectly as
inversion) but disrupts object recognition more than inversion
does. Both manipulations were used, because each has advantages that the other lacks. Each method succeeded in disrupting
recognition; compared with Exp 1 and 2, RTs were slower in Exp
3 and 4, and accuracy was worse overall in Exp 4 (SI Appendix
1.3). If lower-level characteristics were causing the animate
attention effect, then it should still appear in Exp 3 and 4. It did
not (see Fig. 4).
Specifically, inverting scenes eliminated the animate advantage in detection speed (P ⫽ 0.25). Changes to inverted people,
animals, fixed artifacts, and plants elicited comparable mean
detection times (Fig. 4A; SI Appendix 1.4). When inverted,
accuracy was comparable for fixed artifacts, plants, people, and
animals. (Compared with other inanimate targets, accuracy for
inverted moveable artifacts was disproportionately low, a pattern
not seen in the upright conditions; SI Appendix 1.5). This is in
contrast to the pattern for upright scenes, where animate beings
showed a consistent speed and accuracy advantage compared
with all inanimate categories.
Blurring upright scenes also eliminated the animate advantage
in detection speed (P ⫽ 0.17). There was no animate advantage
in accuracy either. In fact, the reverse was true: in the blur
condition, accuracy was greater for inanimate objects (Fig. 4B;
SI Appendix 1.6).
Inversion and blurring disrupt recognition, which is necessary
for targets to be identified as animate vs. inanimate, while
PSYCHOLOGY
Is Preferential Attention to Animates a Side Effect of Differences in
Lower-Level Visual Characteristics? Does the animate attention
Fig. 4. Disrupting recognition eliminates the advantage of animates in
change detection, showing that the animate advantage is driven by category,
not by lower-level visual features. Graphs show proportion of changes detected as a function of time and category when recognition is disrupted.
(Inset) Mean RT for each category. (A) Results for Exp 3 using inverted stimuli.
RT, animate M ⫽ 5,399 (SD, 2,139), inanimate M ⫽ 5,813 (SD, 2,405). (See SI
Appendices 1.4 and 1.5.) (B) Exp 4, blurred stimuli. RT, animate M ⫽ 5,792 (SD,
2,705), inanimate M ⫽ 5,337 (SD, 2,121). Accuracy; animate M ⫽ 45.2% (SD,
15.1), inanimate M ⫽ 56.7% (SD, 13.5), greater accuracy for inanimates; P ⫽
0.0001, r ⫽ 0.67.
preserving all (inversion) or some (blurring) lower-level stimulus
characteristics. That these two manipulations eliminated the
animate detection advantage found in Exp 1 and 2 indicates
the animate attentional advantage depends on recognition of the
target’s semantic category. It was not a side effect of incidental
differences in contrast, visual complexity, or any other lowerlevel property of the stimuli used. (Additional controls show that
the animate advantage also remains strong when controlling for
potential differences in scene backgrounds; see SI Appendix 1.7).
Is Preferential Attention to Animates a Consequence of Experience
with Motion? The animate monitoring hypothesis proposes that
animates are attended by virtue of category-specific attentional
mechanisms that are triggered by properties of animals and
humans, not by mechanisms that attend to anything often seen
in motion. Vehicles were chosen as a control category, because
they move yet are not animals.
Vehicles are seen in motion every day, and the failure to
monitor that motion has life-or-death consequences. Indeed, this
16602 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0703913104
expertise might give vehicles a detection advantage over other
inanimate objects. But the prediction that animate inputs will be
closely attended is not based on expertise-driven attention. It is
based on the hypothesis that the visual system is designed to
monitor animates because of their importance ancestrally. Consequently, animals and people should be monitored more closely
than vehicles, despite our ontogenetic experience of vehicular
motion and its importance to survival in the modern world. To
test this, we conducted Exp 5, which specifically compared
detection of animate targets to vehicles.
Of the artifact targets, 24 were vehicles (on roads, rivers, etc.),
and 24 were artifacts that do not move on their own (e.g.,
lampposts, keys). To see whether there is any effect of implied
motion on attention (23–25) due not to the target’s category but
rather to representations of motion or momentum (e.g., sitting
vs. walking), half the people and half the animals were in motion,
and half were not. Thus, there were static and dynamic animate
targets and static (lampposts, keys) and dynamic (vehicles)
inanimate targets. Otherwise, the procedure was the same as for
Exp 1.
The results of Exp 5 are shown in Fig. 3C. Accuracy for
vehicles and static artifacts was low (and comparable), with
changes to vehicles detected faster than changes to static artifacts
(P ⫽ 0.00072, r ⫽ 0.52). Nevertheless, changes to animals and
people were detected ⬎1 second faster than changes to vehicles,
and the effect size was large, r ⫽ 0.82 (animate vs. vehicles, P ⫽
10⫺11). Even so, this underestimates the animate attentional
advantage over vehicles, because accuracy for animate targets
was 27% points higher than for vehicles, another large effect size,
r ⫽ 0.87 [animate vs. vehicle, 90.6% (SD, 7.8) vs. 63.5% (SD,
18.8), P ⫽ 10⫺12]. That is, subjects were change blind ⬎36% of
the time for vehicles but ⬍10% of the time for animals and
people. Detection of animate targets was better than vehicle
targets at all time intervals, even ⬍1 second.
Compared with vehicles, the speed and accuracy advantage for
non-human animals was just as large as for people (animals vs.
vehicles, RT, r ⫽ 0.80, P ⫽ 10⫺10; hits, r ⫽ 0.84, P ⫽ 10⫺14; people
vs. vehicles, RT, r ⫽ 0.78, P ⫽ 10⫺9; hits, r ⫽ 0.88, P ⫽ 10⫺16).
Moreover, the advantage for non-human animals remains just as
large if the vehicle category is restricted to include only cars and
trucks, the vehicles that subjects need to monitor most often
(RT, r ⫽ 0.79, P ⫽ 10⫺9; hits: r ⫽ 0.85 P ⫽ 10⫺15).
To make sure these effects were not due to incidental differences in low-level visual characteristics, we conducted an inversion control for Exp 5 (analogous to Exp 3). Although there were
some differences between categories on inversion, the animate
attentional advantage in Exp 5 remains large and significant
when these potential differences in low-level features are controlled for (RT, r ⫽ 0.74, P ⫽ 10⫺7; hits, r ⫽ 0.88, P ⫽ 10⫺12; SI
Appendix 1.8). The same is true when one controls for potential
differences in scene backgrounds (SI Appendix 1.7).
It is known that the human visual system has a bias to detect
motion, and that momentum is represented even from still
pictures (23–25). Are changes to animals and people detected
faster and more accurately merely as a side effect of attention to
objects in motion, animate or not?
No. For the animate monitoring effect to be a side effect of
motion detection, there would have to be a CD advantage for
targets in motion over stationary ones, even for the categories
animal and person. Fig. 3C shows this was not the case; for animals
and people, CD was just as fast and accurate when their pose was
stationary as when it was dynamic (stationary vs. dynamic; hit RT
means 2,660 msec (SD, 968) vs. 2,661 (SD, 1,142); hit rates, 91% for
both). Thus implied motion does not cause a category-independent
increase in attentional monitoring.
Because there were no category-independent effects of representational momentum on change detection, such effects
cannot explain the CD advantage of vehicles over static artifacts.
New et al.
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Conclusion
Changes to animals, whether human or non-human, were detected
more quickly and reliably than changes to vehicles, buildings, plants,
or tools. Better change detection for non-human animals than for
vehicles reveals a monitoring system better tuned to ancestral than
to modern priorities. The ability to quickly detect changes in the
state and location of vehicles on the highway has life-or-death
consequences and is a highly trained ability; indeed, driving provides training with feedback, a situation that should promote the
development of expertise-derived selection criteria. Yet subjects
were better at detecting changes to non-human animals, an ability
that had life-or-death consequences for our hunter–gatherer ancestors but is merely a distraction in modern cities and suburbs. This
speaks to the origin of the selection criteria that created the animate
monitoring bias.
The selection criteria responsible were not goal-derived: the
only instructed goal was to detect changes (of any kind), and
there was nothing in the structure of the task to make animals
more task-relevant than inanimate objects (if anything, the
reverse was true: there were more changes to inanimates than to
animates). Nor were they expertise-derived: in the modern
world, detecting changes in animals is an inconsequential and
untrained ability compared with detecting changes in vehicles.
Taken together, the results herein implicate a visual monitoring
system equipped with ancestrally derived animal-specific selection criteria. This domain-specific subsystem within visual attention appears well designed for solving an ancient adaptive
problem: detecting the presence of human and non-human
animals and monitoring them for changes in their state and
location.
Materials and Methods
Five CD experiments were conducted, each involving a different
set of subjects (SI Appendix 1). The 70 scenes used in Exp 1–4
are shown in SI Appendices 3–7. The 96 scenes used in Exp 5 are
shown in SI Appendices 8–11; there were 48 with artifact targets
and 48 with animate targets (24 people and 24 animals). Of the
artifact targets, 24 were vehicles, and 24 were artifacts that do not
move on their own.
We thank Max Krasnow, Christina Larson, June Betancourt, Jack
Loomis, and Howard Waldow and appreciate the support of the University of California, Santa Barbara (UCSB) Academic Senate (L.C.);
UCSB Graduate Division (J.N.); National Institute of Mental Health
National Research Service Award (F32 MH076495-02, to J.N.); and
National Institutes of Health Director’s Pioneer Award (to L.C.).
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PNAS 兩 October 16, 2007 兩 vol. 104 兩 no. 42 兩 16603
SEE COMMENTARY
PSYCHOLOGY
Is There an Effect of Ontogenetic Expertise? The CD advantage of
vehicles over other inanimate objects is consistent with a modest
expertise effect, although it could also be a side effect of an
animate attention bias that is weakly evoked by vehicles (people
make vehicles move; psychophysically, vehicular motion exhibits
the contingent reactivity of animate motion) (28). But if experience were having a major effect on incidental attention, we
would see a large CD advantage for vehicles over animals.
Instead, the reverse is true. There would also be a large CD
advantage for humans over non-human animals, a prediction
that is also falsified.
In modern environments, encounters with other humans are
more frequent and have greater consequences than encounters
with non-human animals. So how much (or little) does ontogenetic
expertise with humans promote change detection, compared with
non-human animals? The curves for animals and humans are
almost identical in Exp 5 (Fig. 3C), and they track each other closely
for time intervals ⬍3–4 seconds in Exp 1 and its replication (Exp
2). More specifically, in Exp 1, 2, and 5, there was no speed
advantage for humans over animals (animals vs. humans, mean RT
for hits, P ⫽ 0.83, 0.46, 0.07; animals were faster). Accuracy was the
same in Exp 5 (P ⫽ 0.07) but higher for humans than for animals
in Exp 1 and its replication (Exp 1, P ⫽ 0.0003, r ⫽ 0.61; Exp 2, P ⫽
10⫺7, r ⫽ 0.76).
Close attention to non-human animals makes sense in ancestral environments but not in the ontogenetic environment experienced by our subjects. Moreover, subjects are visually
trained on the human species many orders of magnitude more
than on any other species. If expertise acquisition was a function
of frequency of exposure and stimulus importance, then change
detection for human targets should be orders of magnitude
better than for non-human animal targets. Yet there was no
speed advantage for detecting changes to humans, and a lifetime
of exposure to humans led only to an inconsistent advantage in
accuracy: more changes were detected when the target was a
person than an animal in Exp 1 and its replication but not in Exp
5. The limited differences in outcome compared with massive
differences in training indicate that other causes are at play aside
from, or in addition to, training. These results, like the animal–
vehicle difference, call into serious question ontogenetic explanations that invoke only domain-general expertise learning.
EVOLUTION
This suggests that the vehicle vs. static advantage was caused by
greater monitoring of objects identified as vehicles (whether in
motion or not).
Better change detection for non-human animals than for
vehicles demonstrates a category-based dissociation between
recognition and monitoring. In directed categorization tasks, the
visual system can rapidly detect the presence of both animals and
vehicles in a natural scene (26), even in the near absence of
attention (27). But the large difference in change detection
demonstrated here shows that the attentional system spontaneously monitors animals more than vehicles (or other inanimates),
even when there is no instruction to do so.